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Forest tent caterpillars: Pockets of defoliation likely this year, but no regionwide outbreak

Forest insect experts are starting to see more forest tent caterpillars in northern Minnesota and Wisconsin, but not enough to call it a major outbreak.

In Minnesota, the caterpillars are defoliating trees as far north as Detroit Lakes and the east shore of Lake Mille Lacs. But there are no major signs of egg masses or early defoliation in Northeastern Minnesota.

Mike Albers, forest health specialist for the DNR in Grand Rapids, said he found few pockets of egg masses when he looked from Grand Rapids through the Iron Range and into Duluth.

"I found a few in Duluth near areas with a lot of lights that would have attracted moths last summer; and a few near Carlton," Albers said. "But I was expecting to see more. I don't think we'll see any major outbreak. But people in specific areas where there were egg masses, they could see some defoliation. We're already getting some reports of the tops of trees getting thin."

Those eggs, laid last summer by small brown moths, will hatch as hungry caterpillars that eat the leaves of trees, especially aspen. Albers said defoliation will be most noticeable in early June. The caterpillars will then spin a web and turn into moths. The moths will hatch in July to lay more eggs and continue the cycle. So if you see moths in July, be ready for caterpillars in your area next year.

A major outbreak is overdue. Forest tent caterpillars, sometimes wrongly referred to as army worms (that's a different pest altogether) have cyclical explosions in population about every 10 years, although the peaks can range from six to 16 years apart, Albers said. Those outbreaks have been traced back to at least the 1870s.

The last peak defoliation from the caterpillars, and the largest ever, was 7.75 million acres in 2001, with 7.3 million acres of forest defoliated in 2002. Those outbreaks caused a leafless, wormy mess wherever they marched, ruining outdoor events with their squirming, defecating hordes.

Caterpillar numbers then crashed to their low point in 2006, with just 1,000 acres defoliated. Those defoliation numbers have been very slowly moving back up, with most of the affected forests in west-central Minnesota.

Wisconsin Department of Natural Resources forest insect specialist Brian Schwingle told KUWS Radio that he expects a minor forest tent caterpillar outbreak to begin in Burnett and Polk counties this year, but not a major infestation. Smaller outbreaks could occur in the far northern tier of Wisconsin counties as well, Schwingle said.